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WORKER bees and wasps are usually sterile, yet they expend much energy looking after the young produced by other members of the group. Why are the workers apparently so altruistic? William Hamilton, now at Imperial College, London, suggested an answer 25 years ago. Helping the young of others is a form of behaviour that evolves only if the individuals are closely related, he argued. New work has questioned the validity of this theory in some species of wasp. Because workers share many genes with the queen, they work to ensure that some of those genes are passed on to the next generation. Although this explanation – known as ‘kin selection’ – has wide acceptance, some species of wasp appear to pose problems for the theory. The swarming wasps of the neotropics, for example, typically have many queens. The individuals in such colonies should be less closely related than wasps in colonies derived from a single queen, unless, as Hamilton suggested, the whole population has become severely inbred. David Queller and his colleagues at Rice University in Houston, Texas, tested this hypothesis. They collected wasps from three swarming species in Venezuela to estimate the degree of relatedness of individuals in each colony (Science, vol 242, p 1155). The researchers estimated how closely related individuals of each species were to each other by looking at some of the proteins that they produce. They found no evidence for Hamilton’s hypothesis. The inhabitants of some colonies were not closely related, and in all three species, the level of inbreeding was low. The two species of Polybia wasps that the researchers studied have between two and four queens on average. They are not very inbred: the level of inbreeding is about the same as it is in Polistes wasps, which have a single queen. So relatedness alone may not be the only factor in the evolution of worker wasps, at least for these species. The third species, Parachartergus colbopterus,